Non-alcoholic steatohepatitis (NASH) is a severe liver lesion that is characterized by necrosis, inflammation and fibrosis. NASH and alcoholic steatohepatitis (ASH) are histologically indistinguishable and are thought to have the same etiology. Nowadays, NASH is considered to be a progressive disease with 25% incidence of cirrhosis and 10%-15% incidence of mortality. It is also believed to cause liver disease in most cases of cryptogenic cirrhosis (unknown cause). Therefore, research on NASH has become one of the subjects with top priority in clinical hepatology. NASH epidemiology, characteristics, diagnosis strategies and treatment have been reviewed by Andrea E. Reid [Reid A. E., Gastroenterology, 2001, 121:710-723].
To illustrate the problem, some epidemiological data could be enumerated:                between 7% and 11% of the patients who undergo an liver biopsy in the United States and Canada are diagnosed with NASH;        between 60% and 83% of the patients diagnosed with NASH are women;        by 30% of the obese patients suffer from NASH;        fibrosis appearance is detected in 43% of patients with NASH;        cirrhosis incidence in patients with NASH is approximately 25%;        all the patients who develop alcoholic cirrhosis (approximately 50% out of all cirrhosis cases) have previously developed ASH; and        NASH is frequent in patients with hepatitis C.        
Due to the prominent position of NASH among chronic and progressive liver diseases, there is an increasing interest in the determination of its pathogenesis. However, pathophysiological mechanisms leading to NASH development have not been determined yet. Alcoholic people may have a moderately fatty liver for many years and, without a change in their drinking habits, suddenly develop severe alcoholic hepatitis. In the same way, obesity, type II diabetes and hypertriglyceridemia are frequently associated with fat accumulation in liver and, although this situation does not invariably lead to the development of necroinflammatory lesions, these patients are at higher risk to develop NASH. Consequently, the existence of environmental or cellular factors working as inhibitors of a cascade of molecular events inducing necrosis, inflammation and fibrosis have been proposed. Portal endotoxemia and lipid peroxidation are two of these possible factors. Alterations in the expression of different genes/proteins, such as CYP2E1, CYP4A, UCP2, have also been identified in NASH and ASH; however, none of these genes have a value for the diagnosis or prediction of the development of the disease in time.
Nowadays, most NASH patients are examined due to high, chronic results in liver function analysis (e.g., moderate chronic increase of aminotransferases), hepatomegalya, or both. A combination of the clinical history, physical examination, blood analysis and radiological and histological examinations exclude other causes of liver disease. Blood analysis has to include a complete liver profile including, for example, blood cell count and anti-HCV antibodies, hepatitis B surface antigen, hepatic iron index, ceruloplasmin, antinuclear antibody, α1-antitrypsin and anti-mitochondrial antibody determination. Image diagnosis, for example with liver ultrasonography (the preferred modality of image diagnosis), reveals the existence of a “shiny” liver with increased ecogenity. Nevertheless, this technique sensitivity and specificity for steatosis are 89%-95% and 84-93% respectively. Fatty liver diagnosis can also be diagnosed by computerized tomography or by abdominal magnetic resonance. In any case, clinical, analytical and radiological data do not allow the differentiation of NASH and alcoholic hepatitis given that both pathologies are histologically identical (presence of diffuse or centrilobular macrovesicular steatosis, ballooned hepatocytes, necrosis, mixed lobular inflammatory infiltration, with or without necrosis, Mallory bodies, lipogranulomes and glycogenic nuclei). Therefore, the diagnosis of NASH is only confirmed in the cases of significant alcohol intake absence (typically lower than 20-40 g alcohol/day). Up to now, the proposed molecular markers (CYP2E1, CYP4A, UCP2) do not have a value for the diagnosis or prediction of the development of the disease in time.
Therefore, it is necessary to have tools to study NASH progression in time and to identify molecular markers associated with NASH. One of these tools could be an animal model which allowed the analysis of the patterns of genes and proteins in a normal liver and in a liver with NASH, as well as along the disease progression in time. Identification of these molecular markers, particularly early molecular markers, with a diagnosis and predictive value for NASH, and the study of their functional effects, would help prevent and/or treat NASH. Their identification would also help search and develop useful drugs for preventive and/or curative treatment of this disease. A good molecular marker of NASH would appear early in the liver, long before histological alterations were observed. Ideally, to efficiently diagnose the predisposition to suffer from NASH, it would be better to have a group of early molecular markers of NASH appearance than to have only one. This group of molecular markers would be something like the early “fingerprint” of the disease.
In this research line, a model for the in vivo study of NASH has been developed. This model is based on a knockout mouse deficient in MAT1A gene, that is, in S-adenosylmethionine (AdoMet) synthesis in liver, an essential cellular metabolite, named MATO (Lu S. C., et al., Proc. Natl. Acad. Sci. USA, 2001; 98:5560-5565). In addition to NASH, these MATO mice spontaneously develop oxidative stress and hepatocellular carcinoma (Martínez-Chantar M. L., et al., Faseb J., 2002, 16:1292-1294). MAT1A gene is specifically expressed in adult livers, although it has been demonstrated that the expression of this gene is silent in patients with hepatic cirrhosis (both alcoholic and non-alcoholic). 3-month-old MATO mice livers are normal but they are much more liable to develop severe macrovesicular steatosis (induced by a choline-deficient diet) and necrosis (induced by CCl4); at 8 months, MATO mice spontaneusly develop NASH; and at 14-18 months they are at high risk to develop hepatocellular carcinoma (more than 80% of the animals develop hepatic tumours). Preliminary DNA microarray assays (Lu S. C., et al., Proc. Natl. Acad. Sci. USA, 2001; 98:5560-5565) show that at 3 months of age (when the histology of the liver is still normal), there are already hundreds of differences in gene and protein expression between normal or wild type (WT) and MATO mice. That is to say that long before an histological lesion is observed in the liver, NASH has already started to develop at a molecular level, although many of said differences do not remain throughout the progression of the disease.